A precise mask-to-wafer alignment system and method is essential for submicron lithography. A lithographic pattern must be accurately positioned (aligned) relative to prior patterns. The specialized structures used to align the patterns are often referred to as "alignment marks," or more specifically as a "mask-mark" and "wafer-mark."
Preferably, the overlay accuracy between patterns should be less than 1/5 or even less than 1/10 of the feature size. For example, overlay accuracy should be less than 0.1 microns for a 0.5 micron linewidth. A number of optical techniques are in use with varying degrees of success. However, all techniques result in limits on performance which do not fully satisfy the requirements of lithography systems.
A method of aligning objects is disclosed in U.S. Pat. No. 4,849,911 entitled "Method for Aligning First and Second Objects, relative to Each Other, and Apparatus for Practicing this Method", issuing to Uchida et al on Jul. 18, 1989, which is herein incorporated by reference. Therein disclosed is a devise for aligning a mask and wafer having diffraction gratings thereon. The intensity of the diffracted light is detected and the alignment of the mask and wafer determined. The alignment detection is independent of the gap between the mask and wafer.
While these and other alignment systems have performed adequately for their intended purpose, there is an increasing need for improving the accuracy of alignment systems. This need is particularly evident as the feature size of masks become smaller, and their alignment with a wafer becomes critical.